Myopia or "nearsightedness" is most commonly treated by the use of eye glasses or contact lenses. Occasionally, however, a myopic person cannot or does not wish to rely on glasses or contact lenses. In recent years, in limited cases, myopia has been surgically treated by a procedure called a "radial keratomy." In this procedure a series of cuts arranged like the spokes of a wheel are made in the cornea. The cuts produce a flattening of the corneal curvature and, if successful, thereby correct the myopia. The procedure, which was developed in Russia, has met with only limited acceptance by surgeons in the United States.
The radial keratomy method has two drawbacks: (1) it can never have a fully predictable outcome and (2) any non-spherical flattening of the cornea during healing will result in an eyesight defect which cannot be corrected by the use of eye glasses or contact lenses.
A modification of the radial keratomy procedure, called keratoprosthesis, was recently developed in the United States. In this procedure a lamellar keratoplasty is performed in which only the superficial layer of the cornea is removed and replaced with a corneal implant. These implants are a replacement for the natural cornea where the cornea has become opaque-or fogged as may occur as the result of burns. This procedure, if successful, also tends to flatten the cornea and hence remove the cause of the myopia. Like the radial keratomy procedure the lamellar keratoplasty is traumatic to the patient and the patient may experience difficulty during the healing process such as infection, epithelial cell granulations, scar tissue foundation and the like.
After a keratoprosthesis is inserted between the layers of the natural cornea a window is cut out of at least the damaged outer layer to permit the eye to see through the prosthesis. If the inner layer of the cornea is undamaged then it is only necessary to trephine the outer layer of the cornea leaving the aqueous humour separated from the surrounding air by the inner layer of the cornea backed by the keratoprosthesis. Should the inner layer of the cornea also be damaged then it too must be removed leaving the prosthesis as the only layer between the aqueous humour and the ambient air. Thus, the keratoprosthesis is always in contact with the air and is held in place by the portions of the natural cornea which remain only around its periphery.
Choyce, U.S. Pat. No. 4,607,617, describes a method for correcting an eyesight defect in which an implant made from polysulfone plastics material is inserted into a pocket formed in the cornea. This polysulfone lens has a refractive index which is greater than the refractive index of the cornea itself. Thus, Choyce alters the optical power of the cornea by inserting a plastic lens which has a high refractive index. However, the use of a plastic lens does not allow a sufficient amount of the fluid circulating in the cornea to reach the cornea tissue in front of the lens; as a result, this tissue may suffer tissue rot from a lack of nutrition.
It is thus an object of this invention to provide a method for correcting a refractive error in an eye where the method is not characterized by the foregoing problems. It is another object of the invention to provide an improved surgical procedure for the treatment of myopia, hyperopia, astigmatism or aphakia. It is yet another object of the invention is to provide such a treatment for myopia which is less invasive than surgical procedures heretofore practiced. It is still another object of the invention to provide the implements with which the procedure may be practiced.